The origin of substance P (SP)-immunoreactive neurons in the lower respiratory tract, esophagus and heart of guinea-pigs was demonstrated by surgical denervation or capsaicin pretreatment with subsequent determination of the tissue levels of SP by radioimmunoassay. In other experiments the effect of vagal nerve stimulation on the SP levels in these tissues was studied. The effects of capsaicin-sensitive afferents in the respiratory tract mucosa and bronchial smooth muscle was also studied by analysis of vascular permeability to Evans blue and insufflation-pressure changes. Our present data indicate that all SP nerves in the trachea and lung are afferent and capsaicin-sensitive. The trachea and stem bronchi receive SP afferents mainly from the right vagus nerve with cell bodies located in both the nodose and jugular ganglia. The SP innervation of the lung seems to have a dual origin: 1. Afferents from both vagal nerves with a crossed type of innervation pattern. 2. A non-vagal source which consists of about 40% of the SP nerves in the lung. These nerves probably originate from thoracic spinal ganglia. The effects of ether and capsaicin on insufflation pressure and increase in vascular permeability were dependent on the integrity of capsaicin-sensitive afferents of both vagal and non-vagal origin. In the guinea pig, systemic capsaicin pretreatment to adult animals seemed to result in irreversible changes in the respiratory tract, while in the rat a successive recovery of the functional response of capsaicin-sensitive afferents occurred. Different regimes of systemic capsaicin pretreatment induced different effects on the cholinergic (atropine-sensitive) insufflation-pressure response. Capsaicin pretreatment, using multiple injections over two days, depressed the cholinergic insufflation-pressure increase, while the cholinergic vagal component was unaffected in animals which received a single dose of capsaicin or local pretreatment with capsaicin on the vagal nerves. The local treatment was more effective with regard to SP depletion in target areas when using alcohol as solvent than when capsaicin was dissolved in paraffin oil, while the functional deficits were similar. The SP nerves in the esophagus were mainly of vagal afferent origin, while the heart atrium seemed to have a dual innervation by both vagal and non-vagal SP nerves.